Significant prior research has been performed in the area of haptic interfaces. Several haptic-based interaction systems have been developed and used for a variety of applications, including molecular dynamics simulation and steering, manipulation of nano-materials, surgical training, virtual prototyping, and digital sculpting.
Early haptic interface systems utilized a robot arm, both as a six degree-of-freedom input device as well as a force feedback output device, providing the user with a tactile perception of molecular forces and torques. Since then, alternative force-feedback devices with multiple degrees of freedom have been proposed. These approaches provide an intuitive interface for the manipulation of rigid bodies subjected to inertial, contact, or other forces. They are, however, significantly less convenient for sensing and altering the shape of curves and surfaces.
These haptic devices and techniques focus on force feedback, which assists the user in gauging the effort required to be exerted on the surface in order to achieve the desired shape alteration. This approach may also be used to provide information about the stiffness or density of the surface. In addition, such haptic approaches have been applied to the exploration of a field in a volume or even of fluid dynamics. However, these approaches do not provide sufficient tactile feedback regarding the shape of the surface.
Running the tip of a computer cursor over the virtual surface has been suggested as a means for “haptic surface rendering” and have been extended to real-time detection of contacts when manipulating an object with six degrees of freedom. The contact forces may be computed using the concept of “virtual proxy”.
Such approaches, based on exploration of a surface with the tip or side of a stylus, produce forces that would result from contact, palpation, or stroking actions. These forces may reveal surface anomalies or attract the attention of the designer to small, high-spatial-frequency features that may have been more difficult to detect visually. However, stylus-based approaches are far from exploiting the natural ability of a designer to feel a surface by touching it with a wider area of the hand.
Interfaces involving touch have used gloves, manipulators controlling a stylus held by the hand, and arrays of actuators to depict a surface. They attempt to supply sensations received through our various touch and kinesthetic receptors, often broken into several regimes. Vector macro forces are at the gross end of that scale and are readily displayed by manipulator-like haptic devices. Vibrations are by nature a scalar field and may be distributed widely over the surface of the skin. The amplitude and frequency are noticeable but not the direction. The most difficult to display are small shapes, for which arrays of stimulators are necessary. To achieve both kinesthetic and tactile sensations simultaneously the combination of a haptic manipulator and a tactile array is currently required.
A stylus grasped by a user is one way to explore a haptic environment in a pointwise fashion. If the stylus is attached to a manipulator, interaction forces can be generated which represent interaction of the stylus with a virtual world. Available pointwise haptic displays allow forces and moments to be fed back to the user in two to six degrees of freedom and are well suited to provide the kinesthetic portion of a haptic experience. Haptic mice enable the user to feel the transition of the cursor between different regions of the screen. These haptic manipulators open new possibilities of interfacing, but are comparable to displaying a picture to a viewer one pixel at a time. Haptic manipulators must provide spatial relationships only through temporal sequencing, greatly compromising their efficiency. Sample rates of 1000 Hz are typical with forces controlled at 30 Hz or more for adequate display of features such as a breast tumor.
It is necessary to provide a totally synthetic view of the hand in the environment if haptics are coordinated with vision. Viewing the stylus and its device provides no supporting optical illusion. Another disadvantage of the numerous devices is that the ratio of the smallest to the largest displayable force is difficult to expand. When the hand should be moving unimpeded, it still must exert a force to move the device forward. This problem has been only partially overcome by utilizing a servomechanism based on the position of the hand to avoid contact (i.e., achieve 0 force) except when contact should be displayed.